API: Support string descriptors for unique indexes in Versioned

- Document the format for descriptor arrays
- Implement the behaviour that developers have come to expect for
  string descriptors of indexes
- Add test for handling of unique indexes (MySQL & sqlite3)
- Resolve #2403

Versioned needs to convert unique indexes to non-unique for its suffixed
tables, such as Foo_Live and Foo_versions. Because DataObject accepts
string descriptors such as array('UniqIDX' => 'unique (Uniq)') as well
as array-based descriptors, Versioned needs to recognize string
descriptors. This patch accomplishes that. Before, Versioned would fail
to convert string-described indexes to non-unique, resulting in run-time
errors when creating a new version of an object.
Merge pull request #2686 from oddnoc/indexes-documentation

API: Support string descriptors for unique indexes